Summary of Amplitude Units:

In the English system of measurements, displacement is usually measured in mils (thousandths of an inch), and the peak-to-peak value is used by convention.

Velocity is usually measured in inches per second, and the convention is to use the peak value or the RMS value. The peak value is the most commonly used, not because it is better, but because of long tradition.

Acceleration is usually measured in Gs, where 1 G is the acceleration due to gravity at the earth's surface. The G is not actually an acceleration unit -- it is simply an amount of acceleration we experience as inhabitants of the earth. Acceleration is sometimes measured in inches per second per second (in/sec2), or m/sec2, that are true units. One G is equal to 386 inches/sec2 or 9.81 meters/sec2.

The process of converting a signal from displacement to velocity or velocity to acceleration is equivalent to the mathematical operation of differentiation. Conversely, the conversion from acceleration to velocity or velocity to displacement is mathematical integration. It is possible to perform these operations in vibration measuring instruments and thus to convert from any system of units to any other one. From a practical standpoint however, differentiation is an inherently noisy process, and is seldom done. Integration, on the other hand, can be done very accurately with inexpensive electrical circuitry. This is one reason that the accelerometer is the de facto standard transducer for vibration measurement, for its output is easily integrated once or twice in order to display velocity or displacement. Integration is not suitable, however, for signals of very low frequencies (Below 1 Hz), for in this region the noise level increases and the accuracy of the integration process itself suffers. Most commercially available integrators operate correctly above one Hz, which is sufficiently low for almost all vibration applications.

DLI Engineering provides products and services for Predictive Maintenance including vibration analysis instruments, monitoring and diagnostic software, and consulting for CBM programs.

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